Typical impact speeds are about 20km per second (70,000km/h). Asteroids smaller than about 50m do not reach the ground -- but
blow up in the atmosphere, although the blast can still be very dangerous.
An example occurred in 1908 over Siberia. Larger asteroids reach the ground at
very high speed and produce large craters. We know of more than 20 craters in
Australia alone. However, this is not a case of the earth being blown up. The
earth has been hit many times in the past, with many extinction events such as
the dinosaurs, and the thing we are worried about is not saving the world, but
saving people.

Dr. Ducan Steele, Australian Sixty Minutes

"Current detection and deflection strategies involve the
assumption
that decades or centuries of warning will be available following
the discovery of a threat asteroid. However if the major impact
hazard indeed comes from this essentially undetectable population,
the warning time of an impact is likely to be at most a few days. A
typical Halley-type dormant comet spends 99 per cent of its time
beyond the orbit of Mars and so a full mapping of this population is beyond
current technology."

While I believe that the
British neo-catastrophists are wrong about the threat to Earth, their work
is science, not pseudoscience. They are making their case to other scientists,
and time will sort out who is right and who is wrong. Regardless of whether the
specific theories referred to in this article turn out to be correct, observing
comet debris hitting Earth's atmosphere now seems to take on a whole new
perspective in our "enlightened ages.

-- David Morrison, Director of Space, NASA-Ames Research Center, 1997

To Peiser's assertion that, "Some 100 surface impacts,
including more than a dozen oceanic impacts, have repeatedly devastated whole
regions, small countries and early civilizations around the globe" during the
past 10,000 years, Clark Chapman, an asteroid expert at the Southwest Research
Institute in Colorado, wrote that this reflects "historical anecdotes and the
ancient versions of 'urban legends.' I regard these as having little probative
[substantiating] value, unless accompanied by sound physical and geological
evidence. But there are some neo-catastrophists, located mainly in Britain, who
have an almost Velikovskian pseudo-scientific take on this matter and have
argued that such impacts are more frequent..." Velikovsky, of course, is the guy
who gave asteroid impacts such a bad name back in 1950. Benny Peiser
and Clark Chapman on impact frequency, pg. 3 here